For many years, man has extracted oil and gas from beneath the earth's surface. Much of the drilling has taken place in the offshore area where large reservoirs have been discovered beneath the world's seabeds. Various types of structures have been employed in these offshore operations. Most structures include a horizontal working platform or deck which is supported at a safe distance above the water's surface by various support devices.
Some support devices consist of floating members which are kept in position by a plurality of judiciously placed anchors. Other support devices are submerged and attached temporarily to the seabed and are capable of being withdrawn and used at other drilling sites. Still other support devices are permanently attached to the seafloor.
One popular arrangement for permanently attaching the structure to the seafloor involves the use of very long piles which are driven deeply into the seafloor. As an acid in driving these piles, a structure known as a jacket is used. The jacket is a structure containing a plurality of hollow pile sleeves. The pile sleeves serve as guides for driving the piles. The piles are lowered through the sleeves down to the seafloor and are driven into the seafloor. The jacket assures that each pile will be properly placed with respect to one another. In many offshore structures, the piles extend upward, through and beyond the sleeve to a point above the water's surface where they act as a stationary "island" for directly supporting the work deck. In some offshore structures, the work deck may be supported indirectly by the piles, the work deck being attached to members which are in turn attached to the pile sleeves. The piles are usually attached to the pile sleeves by grouting the outer surface of the pile to the inner surface of the pile sleeve. In addition to the pile sleeves, a jacket contains many horizontal, vertical and diagonal members which ultimately serve to provide support for the piles against lateral loads.
Almost all offshore jackets are constructed onshore. After construction has been completed, the jacket is towed to the drilling site and lowered to the proper position on the seabed. Some offshore areas have a very soft, unconsolidated seafloor. An excellent example of this is the Gulf of Mexico around the mouth of the Mississippi River.
The unconsolidated nature of this deltaic area presents challenging problems related to jacket installation. After the jacket has been lowered to the seafloor, it is very difficult to perform the pile driving operations. This difficulty arises from the fact that during the driving of a pile, the jacket tends to sink into the soft mud on that side of the jacket immediately surrounding the pile which is being driven. The tendency to sink can be understood, perhaps, by thinking of an inflated, doughnut-shaped rubber tube which is floating on water. (In this analogy, the tube represents the jacket and the water represents the unconsolidated seafloor). If a weight is placed on one side of the tube (in our analogy, the side of the jacket where the pile driving operation is occurring), that side of the tube sinks down and the other side of the tube tends to come up.
The above described difficulty involving the unconsolidated seafloor has been alleviated in the past by equipping the jacket bottom with devices which are commonly called mudmats. Mudmats are to these offshore structures what snowshoes are to man. These mudmats have a very large area, and, distributing the load of the jacket over said large area, allow the jacket to stand on the soft bottom and provide stability during pile driving operations.
Mudmats, however, in helping to solve this problem have created another problem. Occasionally, the unconsolidated sea bottom shifts position. This shifting creates huge mudslides which move along the bottom of the seafloor and crash into the jacket and the mudmats. The mudmats, having a very large area, are subjected to the tremendous forces exerted by the mudslides. Because the mudmats are attached to the jacket, the forces created by the mudslides are transferred to the jacket. This usually means that the jacket must be built to withstand such a mud load, requiring larger structural members and hence a more expensive jacket. This increase in cost may make an otherwise marginally attractive venture uneconomical.
Currently, as an alternative to increasing the size of the jacket members, the mudmats are removed after jacket installation, i.e. pile driving operations, have been completed. Unfortunately, the typical mudmat is designed such that it is often very difficult and expensive to remove the mudmats from the jacket.
Conventional mudmat design involves attaching the mudmat to the jacket in a manner and location such that the upward vertical projection of the mudmat intersects one or more members of the jacket. Stated another way, the mudmat cannot be removed by lifting it vertically because there are jacket members located above it which would interfere with such an operation. A typical mudmat design is illustrated in U.S. Pat. No. 3,638,436 in FIG. 6, item 17. See also U.S. Pat. No. 3,592,012, FIG. 2, item 34.
Typical mudmat removal includes the use of divers who must be sent to the seafloor. Usually, the mudmat is first cut by the divers into smaller-sized pieces which are easier to handle. For the reasons described above, said pieces cannot be removed by lifting them vertically. Instead each piece must be pulled horizontally until its upward vertical projection is without the upward vertical of the jacket. After a particular piece of the mudmat has been pulled horizontally as described hereinabove, it is hoisted vertically, removing it from the seafloor. This procedure is repeated over and over again for every piece of the mudmat until all the pieces have been removed.
Obviously this method of removing the mudmat entails considerable expense in terms of the amount of diving time required for this job. Futhermore, there are additional costs involving equipment which must be employed on the seafloor to pull each piece of the mudmat horizontally prior to pulling it vertically.